Capping Layers for Improved Crystallization

ABSTRACT

Techniques for fabrication of kesterite Cu—Zn—Sn—(Se,S) films and improved photovoltaic devices based on these films are provided. In one aspect, a method of fabricating a kesterite film having a formula Cu 2−x Zn 1+y Sn(S 1−z Se z ) 4+q , wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; and −1≦q≦1 is provided. The method includes the following steps. A substrate is provided. A bulk precursor layer is formed on the substrate, the bulk precursor layer comprising Cu, Zn, Sn and at least one of S and Se. A capping layer is formed on the bulk precursor layer, the capping layer comprising at least one of Sn, S and Se. The bulk precursor layer and the capping layer are annealed under conditions sufficient to produce the kesterite film having values of x, y, z and q for any given part of the film that deviate from average values of x, y, z and q throughout the film by less than 20 percent.

FIELD OF THE INVENTION

The present invention relates to improving properties of inorganic filmshaving copper (Cu), zinc (Zn), tin (Sn) and at least one of sulfur (S)and selenium (Se) and more particularly, to techniques for fabricationof kesterite Cu—Zn—Sn—(Se,S) films and improved photovoltaic devicesbased on these films.

BACKGROUND OF THE INVENTION

The widespread implementation of next generation ultra-large scalephotovoltaic technologies (beyond 100 gigawatt peak (GWp)) will requiredrastically reducing production costs and achieving high efficiencydevices using abundant, environmentally friendly materials. Thin-filmchalcogenide-based solar cells provide a promising pathway to costparity between photovoltaic and conventional energy sources. Currently,only Cu(In,Ga)(S,Se)₂ and CdTe technologies have reached commercialproduction and offer over 10 percent power conversion efficiency. Thesetechnologies generally employ (i) indium (In) and tellurium (Te), whichare relatively rare elements in the earth's crust, or (ii) cadmium (Cd),which is a highly toxic heavy metal.

Copper-zinc-tin-chalcogenide kesterites, with the ideal formulaCu₂ZnSn(S,Se)₄ (CZTSSe), more generally expressed asCu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q), wherein 0≦x≦1; 0≦y≦1; 0≦z≦1;−1≦q≦1, have been investigated as potential alternatives because theyare based on readily available and lower cost elements. However,photovoltaic cells with kesterites, even when produced using high costvacuum-based methods, had until recently achieved at best only 6.7percent power conversion efficiencies, see H. Katagiri et al.,“Development of CZTS-based thin film solar cells,” Thin Solid Films 517,2455-2460 (2009).

U.S. Patent Application Publication No. 2011/0094557 A1 filed by Mitziet al., entitled “Method of Forming Semiconductor Film and PhotovoltaicDevice Including the Film,” (hereinafter “U.S. Patent ApplicationPublication No. 2011/0094557 A1”) and T. Todorov et al.,“High-Efficiency Solar Cell with Earth-Abundant Liquid-ProcessedAbsorber,” Adv. Mater. 22, E156-E159 (2010), describe a hydrazine-basedapproach for depositing homogeneous chalcogenide layers from mixedslurries containing both dissolved and solid metal chalcogenide speciesdispersed in systems that do not require organic binders. Upon anneal,the particle-based precursors readily react with the solution componentand form large-grained films with good electrical characteristics anddevice power conversion efficiencies as high as 10%.

However, favorable electronic properties of these materials are found ina relatively narrow compositional range, i.e., Cu/(Zn+Sn)=0.7-0.9 andZn/Sn=1-1.3. See, for example, H. Katagiri et al., “Development ofCZTS-based thin film solar cells,” Thin Solid Films, 517, 2455-2460(2009).

A common challenge found in kesterite layer fabrication is the volatilenature of film constituents at high temperature, such as tin (Sn)chalcogenide compounds. See, for example, D. B. Mitzi et al., “The pathtowards a high-performance solution-processed kesterite solar cell,”Solar Energy Materials & Solar Cells, 95, 1421-1436 (2011). Thisproperty makes it particularly difficult to fabricate films withdesirable composition and large-grained structure at elevatedtemperatures.

In addition to Sn compounds, chalcogenides (sulfur (S) and selenium(Se)) are volatile at relatively low temperatures. Their beneficialeffect on film crystallization has been known in CIGS films. See U.S.Patent Application Publication No. 2007/0092648 A1, filed by Duren etal., entitled “Chalcogenide Solar Cells.” This approach of extrachalcogen in the film can be extended to CZTS and has been applied inthe teachings of U.S. Patent Application Publication No. 2011/0094557 A1and U.S. Patent Application Publication No. 2011/0097496 A1, filed byMitzi et al., entitled “Aqueous-Based Method of Forming SemiconductorFilm and Photovoltaic Device Including the Film” (hereinafter “U.S.Patent Application Publication No. 2011/0097496 A1”) for all of the 5deposited layers. Yet excess of chalcogen in the bulk of the film maylead to the occurrence of voids and cracks.

There are reports employing an anneal atmosphere containingtin-sulfur/selenium vapor in a sealed glass ampoule, including a deviceefficiency of 5.4%. See, for example, A. Redinger et al., “TheConsequences of Kesterite Equilibria for Efficient Solar Cells,” J. Am.Chem. Soc., 133 (10), pp 3320-3323 (2011). However, precise processcontrol in this configuration may not be straightforward in large-areaapplications.

Thus, improved techniques for the fabrication of kesterite layers wouldbe desirable. In particular, improved techniques for controlling theconcentration and gradient of volatile elements Sn, S and Se within thebulk CZTSSe film are required in order to target improved deviceperformance.

SUMMARY OF THE INVENTION

The present invention provides techniques for fabrication of kesteriteCu—Zn—Sn—(Se,S) films and improved photovoltaic devices based on thesefilms. In one aspect of the invention, a method of fabricating akesterite film having a formula Cu_(2−x)Zn_(1+y)Sn(S_(z) Se_(z))_(4+q),wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; and −1≦q≦1 is provided. The method includesthe following steps. A substrate is provided. A bulk precursor layer isformed on the substrate, the bulk precursor layer comprising Cu, Zn, Snand at least one of S and Se. A capping layer is formed on the bulkprecursor layer, the capping layer comprising at least one of Sn, S andSe. The bulk precursor layer and the capping layer are annealed underconditions sufficient to produce the kesterite film having values of x,y, z and q for any given part of the film that deviate from averagevalues of x, y, z and q throughout the film by less than 20 percent.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology forfabricating a kesterite film according to an embodiment of the presentinvention;

FIG. 2 is a cross-sectional diagram illustrating a starting structurefor fabricating a photovoltaic device, e.g., a substrate formed from aconductive material or a substrate coated with a layer of conductivematerial according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a kesterite filmabsorber layer having been formed on the substrate according to anembodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating an n-typesemiconducting layer having been formed on the kesterite film and a topelectrode having been formed on the n-type semiconducting layeraccording to an embodiment of the present invention;

FIG. 5 is a surface scanning electron micrograph (SEM) image of abase-line film prepared without using a capping layer according to anembodiment of the present invention;

FIG. 6 is a surface SEM image of a film prepared using a capping layeraccording to an embodiment of the present invention;

FIG. 7 is a cross-sectional SEM image of a completed baseline deviceusing the film of FIG. 5 (without a capping layer) according to anembodiment of the present invention;

FIG. 8 is a cross-sectional SEM image of a completed device using thefilm of FIG. 6 (with a capping layer) according to an embodiment of thepresent invention;

FIG. 9 is an efficiency distribution of samples prepared with no cappinglayer compared with samples prepared using the present techniques havingcapping layers containing tin (Sn) sulfur (S) and selenium (Se)according to an embodiment of the present invention;

FIG. 10 is a surface SEM image of a film prepared from a solution withreduced Se content without using a capping layer according to anembodiment of the present invention;

FIG. 11 is a surface SEM image of a film prepared from a solution withreduced Se content using a Se capping layer according to an embodimentof the present invention;

FIG. 12 is a surface SEM image of a film prepared from a solution withreduced Se content using an SnS—Se capping layer according to anembodiment of the present invention;

FIG. 13 is a surface energy dispersive spectrometer (EDS) scan of asample film prepared without using a capping layer according to anembodiment of the present invention;

FIG. 14 is a surface EDS scan of a sample film prepared using a Sn—Secapping layer according to an embodiment of the present invention;

FIG. 15 is a cross-sectional EDS scan of the sample film preparedwithout using a capping layer according to an embodiment of the presentinvention; and

FIG. 16 is a cross-sectional EDS scan of the sample film prepared usingthe Sn—Se capping layer according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for improving morphology and bulkcomposition of films composed, for example, of copper (Cu), zinc (Zn),tin (Sn), sulfur (S) and/or selenium (Se), including those having akesterite crystal structure, e.g., Cu₂ZnSnS₄ or Cu₂ZnSnSe₄, through theuse of a solid capping layer containing Sn and or a chalcogen (S or Se)which serves to compensate for loss of these elements during thehigh-temperature reactions (e.g., which typically occurs at temperaturesabove 400 degrees Celsius (° C.)) of kesterite crystallization. The useof a solid capping layer, rather than a vapor-phase addition process,can be advantageous because the solid layer may provide at leasttemporarily molten Se-rich compositions during the high-temperaturetreatment that may assist in recrystallizing the chalcogenide-based thinfilm (leading to improved grain structure) and, as well, act as atemporary capping layer to reduce the probability of elemental lossduring the high temperature treatment (leading to better phase purity inthe film). For a general discussion on kesterite and use of kesterite insolar cells, see for example, Schorr, “The crystal structure ofkesterite type compounds: A neutron and x-ray diffraction study,” SolarEnergy Materials and Solar Cells, vol. 95, 1482-1488 (2011), the entirecontents of which is incorporated by reference herein.

FIG. 1 is a diagram illustrating an exemplary methodology 100 forfabricating inorganic films having copper (Cu), zinc (Zn), tin (Sn) andat least one of sulfur (S) and selenium (Se), such as kesteriteCu—Zn—Sn—(Se,S) films. In the following description, when components aredefined as containing elements, it is to be understood that theseelements can be present in either isolated or compound form, (e.g., aSn-containing component can comprise Sn, SnS, SnS₂, SnO, Sn(OH)₂ or anyother known Sn compound).

The desired end product of the process is a kesterite film of theformula

Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q),  (1)

wherein 0≦x≦1; 0≦y≦1; 0≦z≦1; and −1≦q≦1, e.g., wherein x, y, z and qrespectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; −0.5≦q≦0.5. To begin theprocess, a bulk precursor layer is formed on a given substrate. The term“bulk” refers to the fact that the components of this layer will make upthe bulk or majority of the kesterite film, with the capping layer (tobe formed below) serving primarily to replace elements lost from thebulk precursor layer during the high temperature crystallization anneal.The term “precursor” refers to the fact that the final compositionand/or distribution of elements throughout the kesterite film will beestablished only after the capping layer is in place and acrystallization anneal has been performed. Thus, the elements as theypresently exist in the bulk layer are at this stage merely precursorsfor the final kesterite film formation.

According to an exemplary embodiment, the bulk precursor layer willcontain at least some of each of the elements that will make up theend-product kesterite film. Thus, in this example, given Formula 1above, the bulk precursor layer may contain Cu, Zn, Sn and at least oneof S and Se (and in some cases may itself form a kesterite phasematerial, see below). Alternatively, the bulk precursor layer may haveonly some of the component elements, e.g., only Cu, Zn, S and/or Se (noSn). The Sn in this case will be provided by the capping layer. Further,since (as will be described in detail below) the capping layer willcontain Sn, S and/or Se to supplement and/or provide the Sn, S and/or Selost during the final anneal, the bulk precursor layer will be thesource of the Cu and Zn present in the final film.

There are two main advantages of the capping layer. First, as mentionedabove, the capping layer serves to compensate for the elemental loss dueto evaporation (especially critical for the metal ratio, i.e., the Sncontent) during the final anneal. Second, the capping layer providesadditional elemental chalcogen at the surface of the film that promotescrystallization without forming voids in the bulk of the layer, incontrary to excess chalcogen added in the bulk layer(s).

As will be described in detail below, the present techniques may be usedto form a kesterite absorber layer for a photovoltaic device. In thatcase, suitable substrates include, but are not limited to, a metal foilsubstrate, a glass substrate, a ceramic substrate, aluminum foil coatedwith a (conductive) layer of molybdenum, a polymer substrate, and anycombination thereof. For photovoltaic device applications, it ispreferable that the substrate is coated with a conductive coating/layer(such as a molybdenum layer) if the substrate material itself is notinherently conducting. The conductive coating/layer or substrate can, inthat instance, serve as an electrode of the device.

The bulk precursor layer may be formed by depositing its constituentelements (i.e., Cu, Zn, optionally Sn and at least one of S and Se)either all together or sequentially. As shown in FIG. 1, the bulkprecursor layer can be configured in a number of different ways. By wayof example only, in step 102, the bulk precursor layer is made up of alayer of mixed elements. As highlighted above, the bulk precursor layermay contain at least some of each of the elements that will make up theend-product kesterite film. Thus, in this case, the bulk precursor layerincludes Cu, Zn, Sn and at least one of S and Se. To form the bulkprecursor layer having each of these elements, one or more of Cu, Zn,Sn, S, Se, binary metal chalcogenides such as Cu₂S, Cu₂Se, Cu₂(S,Se),SnS, SnSe, Sn(S,Se), SnS₂, SnSe₂, Sn(S,Se)₂, ZnS, ZnSe, Zn(S,Se),ternary metal chalcogenides such as Cu₂SnS₃, Cu₂SnSe₃, Cu₂Sn(S,Se)₃,quaternary metal chalcogenides such as Cu₂ZnSnS₄, Cu₂ZnSnSe₄ andCu₂ZnSn(S,Se)₄, and combinations including at least one of the foregoingelements/metal chalcogenides may be dissolved or dispersed as particlesin a suitable solvent (such as hydrazine or a hydrazine-water mixture,with hydrazine content from about 0.1% to about 99.9%.) and thendeposited on the substrate to form the mixed element bulk precursorlayer. Any suitable deposition process known in the art may be used,including, but not limited to, solution coating, evaporation,electrochemical deposition and sputtering.

In one exemplary embodiment, the bulk precursor layer is formed usingthe techniques described in U.S. Patent Application Publication No.2011/0094557 A1 and U.S. Patent Application Publication No. 2011/0097496A1, the entire contents of each of which are incorporated by referenceherein. By way of example only, using the techniques described in U.S.Patent Application Publication No. 2011/0094557 A1 and U.S. PatentApplication Publication No. 2011/0097496 A1, a solution A would beprepared containing hydrazine, Cu and at least one of S and Se(depending on the desired final composition), a dispersion B would beprepared containing hydrazine, Sn, Zn and at least one of S and Se(depending on the desired final composition), the components A and Bwould be mixed together to form an ink and the ink would be depositedusing any suitable solution-based deposition process (including, but notlimited to, spin coating, dip coating, doctor blading, curtain coating,slide coating, spraying, slit casting, meniscus coating, screenprinting, ink jet printing, pad printing, flexography, and gravureprinting). Techniques for forming an ink are also described in U.S.patent application Ser. No. ______, filed herewith on the same day of______, entitled “Process for Preparation of Elemental ChalcogenSolutions and Methods of Employing Said Solutions in Preparation ofKesterite Films,” designated as Attorney Reference NumberYOR920110410US1, and in U.S. patent application Ser. No. ______, filedherewith on the same day of ______, entitled “Particle-Based PrecursorFormation Method and Photovoltaic Device Thereof,” designated asAttorney Reference Number YOR920110412US1, the entire contents of eachof which are incorporated by reference herein. After a brief heattreatment, the result is a bulk precursor layer of mixed elements. Toincrease a thickness of the bulk precursor layer, the above process ofdeposition and short heat treatment may be repeated until the desiredthickness is attained. The short heat treatment prevents successivelayers from being dissolved when the next layer of solution depositionis performed. The temperature of this short (intermediate) heattreatment is preferably less than the final anneal (see, for example,step 108 described below).

At this point in the process, a composition of the bulk precursor layermay be outside of the compositional range of stability for the kesteritephase. In the most extreme example, the film may contain only Cu, Zn, Sand/or Se (no Sn). In this case the precursor film is not kesteriteuntil the Sn is provided by the capping layer (see below). A kesteritephase implies a certain crystal structure, which is stable only over alimited range of Cu—Zn—Sn—S—Se stoichiometry and heat treatmentconditions. Alternatively, the bulk precursor layer may already at thisstage have the correct stoichiometry and may be heat treatedsufficiently (i.e., enough to stabilize the kesterite phase and enablegrain growth) to yield the kesterite phase (although the heat treatmentconditions might not be optimal, for example, as related to the heattreatment time and duration parameters given below for the final anneal(see description of step 108)). However, the bulk precursor layer wouldeither, for example, not yet have an optimal composition within thekesterite phase for high performance or, for example, would have anoptimal composition but still be susceptible of losing volatile elementsfrom its surface during the final anneal (see description of step 108).A purpose of the capping layer and final anneal (see below) then is toimprove on the final film bulk and surface composition, grain structureor homogeneity).

When, as described immediately above, the bulk precursor layer alreadyhas the correct stoichiometry and has been heat treated to yield thekesterite phase, the bulk precursor layer is referred to herein ashaving a “nominal” kesterite phase and configuration. The term “nominal”means that the film is predominantly the kesterite phase (there maystill be impurity phases present in the film at this point, given slightvariation away from ideal kesterite composition and/or heat treatmentconditions). This can be improved upon during the capping and heattreatment process described below.

Alternatively, in step 104, the bulk precursor layer is composed ofmultiple layers. Specifically, in this example, the bulk precursor layeris actually made up of a plurality of individual layers deposited in astack on the substrate. Each of the individual layers contains one ormore precursor components that will be used to form the kesterite film.Using a simple example (wherein as described above the bulk precursorlayer contains at least some of each of the elements that will make upthe end-product kesterite film), the stack of layers could include onelayer containing Cu, one layer containing Zn, one layer containing Snand another layer(s) containing S and/or Se. Since the components in thelayers will be interdiffused during the anneal, the particular order inwhich the layers appear in the stack may not be important. In somecases, however, one may want to put the least volatile elements on topto reduce evaporation during the heat treatment and/or to place theelement on the bottom of the stack that is least reactive with the backcontact material in the device. The thickness of the layers can betailored based on a desired amount of the given precursor in the finalproduct. For instance, if it is desirable to have twice as much Cu asZn, then the Cu layer can be deposited to about twice the thickness ofthe Zn layer. The layers can be deposited on the substrate using anysuitable deposition process, including, but not limited to solutioncoating, evaporation, electrochemical deposition and sputtering.Alternatively, rather than each layer containing a single element, oneor more layers in the stack may include, e.g., any of the binary metalchalcogenides and/or ternary metal chalcogenides provided above.

In one exemplary embodiment, individual layers for use in forming thebulk precursor layer are deposited using the electroplating techniquesdescribed in U.S. patent application Ser. No. 12/878,746, filed by Ahmedet al., entitled “Structure and Method of Fabricating a CZTSPhotovoltaic Device by Electrodeposition,” (hereinafter “U.S. patentapplication Ser. No. 12/878,746”) the entire contents of which areincorporated by reference herein. U.S. patent application Ser. No.12/878,746 discloses electrodeposition of components of an absorberlayer as distinct layers in a stack, and then forming the absorber layerthrough the use of an anneal step(s) to interdisperse the materialsthroughout the layers.

According to an exemplary embodiment, whichever bulk precursor layerconfiguration is employed (i.e., mixed element or distinct layers), thecomposition of the bulk precursor layer approximates the desired finalcomposition of the kesterite film as per Formula 1, above. By way ofexample only, the bulk precursor layer preferably is within 50%tolerance or less, and more preferably within 20% tolerance of thevalues for x, y, z and q given in Formula 1 for the targeted compositionof the kesterite film. The values of x, y, z and q in the bulk precursorlayer are achieved by controlling the ratio of the elements either inthe solutions or dispersions used for liquid-based deposition or by theratio of elements deposited on the substrate using electroplating,solution coating, vacuum-based deposition, evaporation and sputteringtechniques. Additionally, control over composition in the bulk precursorlayer is provided by what temperature, time, atmosphere conditions areused during the heat treatment conditions used before depositing thecapping layer (e.g., higher temperature, longer times and with an openflow of gas during this heat treatment will lead to more loss of Sn andS and/or Se compared to lower temperature, shorter times and a closedatmosphere). By way of example only, the bulk layer is formed to athickness of from about 100 nanometers (nm) to about 5 micrometers (μm),for example, from about 500 nm to about 3 μm, e.g., from about 1,000 nmto about 2.5 μm.

In step 106, a capping layer that contains at least one of Sn, S and Se]is formed on the bulk precursor layer. Thus, the capping layer is from0% to 100% S, from 0% to 100% Se and/or from 0% to 100% Sn. According toan exemplary embodiment, the capping layer is from about 50% to about90% Se, from about 1% to about 50% S and/or from about 1% to about 50%Sn. The targeted composition of Sn, Se, S, as well as the thickness ofthe capping layer is determined, in part, by the composition of the bulkprecursor layer (as determined, for example, by RutherfordBackscattering or RBS, Medium Energy Ion Scattering or MEIS, EnergyDispersive Spectroscopy or EDS, and X-ray Florescence or XRF). Forexample, if the bulk precursor layer is primarily deficient in Sn, thenmore Sn-compound will be used in the capping layer. In general, at leastenough S or Se will be included in the capping layer to accommodate Snin the +4 valence state (i.e., providing a 2:1 ratio of Se or S to Sn)in this layer. This is done in order to avoid pulling S or Se from thebulk precursor layer to convert the Sn to the +4 state during the heattreatment process (which would then possibly lead to a S or Se deficientfinal film). Additional chalcogen can also be added to the Sncomposition in the capping layer (or alone without any Sn composition ifthe bulk precursor layer is already which is believed to help promotelarge grain size through recrystallization of the film. Note that ifexcess Sn, Se or S is provided in the capping layer (beyond what isrequired to satisfy the stoichiometry requirements of the kesteritefilm), the excess may be removed by heating up the film to a sufficienttemperature (so that these elements have a sufficient vapor pressure)and for a sufficient time to evaporate the excess from the surface ofthe film. Nevertheless, ideally the thickness of the capping layer andparticularly the amount of Sn (least volatile element) is maintained ata low enough level that the majority of Sn can either incorporate intothe film, or effectively maintain sufficient vapor pressure tocompensate for the typically observed Sn evaporation loss during thehigh-temperature anneal step.

According to an exemplary embodiment, to form the capping layer,appropriate amounts of S, Se and/or Sn (or compounds thereof, e.g., SnS,SnSe, Sn(S,Se), SnS₂, SnSe₂, Sn(S,Se)₂) are dissolved in a solvent toform an ink. Suitable solvents include, but are not limited to,hydrazine and hydrazine-water mixtures with hydrazine content from about0.1% to about 99.9%. According to an exemplary embodiment, the inksolution is prepared from a solvent containing from about 1% to about40% hydrazine by volume, and from about 1% to about 40% Se, from about0% to about 40% Sn and from about 1% to about 40% S (atomic % for theSn, S and Se). The concentration of the metals and chalcogens in thecapping layer solution is from about 0.01M to about 5 M.

The resulting solution (ink) is then deposited onto the bulk precursorlayer using any conventional deposition process, including, but notlimited to, spin coating, dip coating, doctor blading, curtain coating,slide coating, spraying, slit casting, meniscus coating, screenprinting, ink jet printing, pad printing, flexography, and gravureprinting. By way of example only, the capping layer is formed to athickness of from about 10 nm to about 3 μm, for example, from about 50nm to about 1 μm, e.g., from about 100 nm to about 600 nm.

In step 108, the bulk precursor layer and the capping layer are annealedunder conditions sufficient to form a kesterite film having a nominallyuniform composition throughout. Namely, the annealing step serves tointersperse the elements throughout the layer (or layers) making up thebulk precursor layer and the capping layer. The term “nominallyuniform,” as used herein, means that there is less than a 20% differencein the atomic ratios of elements across the film thickness. This meansthat if one takes the average composition of the film across theabsorber layer thickness (excluding the interfacial layer of, forexample, MoS₂ or MoSe₂ that typically forms at the back of thechalcogenide-based absorber layer), the composition of each element ofthe film at any point in the bulk of the film (excluding the top andbottom 100 nm) deviates by less than 20% from the average value (whichis also the targeted kesterite composition in terms of x, y, z and q).The bulk composition of the film can be determined by, for exampleRutherford Backscattering or RBS, Medium Energy Ion Scattering or MEIS,Energy Dispersive Spectroscopy or EDS and X-ray Florescence or XRF). Theelemental distribution with thickness in the film can be determined by,for example, secondary ion mass spectroscopy (SIMS) or EDS profilingusing transmission electron microscopy (TEM).

The designation of a nominally uniform composition of the kesterite filmwithin 20% after anneal is used to indicate the indiffusion (i.e.,elements diffusing in to the bulk layer from the capping layer) and/orevaporation of elements from the capping layer and may include possiblecomposition gradient within this limit, optionally resulting in band-gapgrading and improved device performance. As described in S. Chen,“Compositional dependence of structural and electronic properties ofCu₂ZnSn(S,Se)₄ alloys for thin film solar cells,” Physical Review B, 83,125201 (2011), the entire contents of which are incorporated byreference herein, the band gap inCu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q) can be controlled, for example,by varying the S:Se ratio in the kesterite material. For z=1 (pure Sematerial), the band gap is approximately 1.0 eV. For z=0 (pure Smaterial), the band gap is approximately 1.5 eV. For intermediate valuesof “z” the band gap can therefore be tailored anywhere in between thevalues of 1.0 eV and 1.5 eV. Consequently, and gap grading can beestablished in the kesterite absorber layer by grading “z” as a functionof depth in the film. The gradation in “z” can be established by choiceof S:Se ratio in the capping layer (e.g., higher S content leads to moreS incorporation in the kesterite film) and the temperature and durationof the heat treatment (or annealing) to which the capping layer and bulklayer are subjected (shorter times and lower temperatures lead to slowerdiffusion of the elements, making it easier to establish a sharpercompositional gradient in the film).

According to an exemplary embodiment, the annealing conditions include atemperature of from about 300 degrees Celsius (° C.) to about 700° C.,e.g., from about 400° C. to about 600° C. for a duration of from about 1second to about 24 hours, for example, from about 20 seconds to about 2hours, e.g., from about 1 minute to about 30 minutes. The anneal ispreferably carried out in an atmosphere including at least one of:nitrogen (N₂), argon (Ar), helium (He), hydrogen (H₂), forming gas andmixtures thereof. This atmosphere can further include vapors of at leastone of: S, Se, Sn and a compound(s) thereof including, but not limitedto H₂S, H₂Se, SnS, SnSe, Sn(S,Se), SnS₂, SnSe₂, Sn(S,Se)₂. Theatmosphere can still further include trace vapors (e.g., less than 1,000parts per million (ppm)) of at least one of water or oxygen, thoughnormally these components would be reduced to the lowest possible levelto avoid excessive oxidation of the sample. The step of thermaltreatment can be carried out either in a confined environment (substrateenclosed) or in an open environment (substrate not enclosed).

The present techniques may be employed in the fabrication of aphotovoltaic device. See, for example, FIGS. 2-4. To begin thephotovoltaic device fabrication process, a substrate 202 is provided.See FIG. 2. As highlighted above, suitable substrates include, but arenot limited to, a metal foil substrate, a glass substrate, a ceramicsubstrate, aluminum foil coated with a (conductive) layer of molybdenum,a polymer substrate, and any combination thereof. Further, as describedabove, if the substrate material itself is not inherently conductingthen the substrate is preferably coated with a conductive coating/layer.This situation is depicted in FIG. 2, wherein the substrate 202 has beencoated with a layer 204 of conductive material. Suitable conductivematerials for forming layer 204 include, but are not limited to,molybdenum (Mo), which may be coated on the substrate 202 usingsputtering or evaporation.

Next, as illustrated in FIG. 3, a kesterite film 302 is formed on thesubstrate 202. In the particular example shown in FIG. 3, the substrate202 is coated with the conductive layer 204 and the kesterite film 302is formed on the conductive layer 204. Kesterite layer 302 may be formedon the substrate 202 using the techniques described in conjunction withthe description of methodology 100 of FIG. 1, above. The kesterite film302 will serve as an absorber layer of the device.

An n-type semiconducting layer 402 is then formed on the kesterite layer302. According to an exemplary embodiment, the n-type semiconductinglayer 402 is formed from zinc sulfide (ZnS), cadmium sulfide (CdS),indium sulfide (InS), oxides thereof and/or selenides thereof, which isdeposited on the kesterite layer 302 using for example vacuumevaporation, chemical bath deposition, electrochemical deposition,atomic layer deposition (ALD), and Successive Ionic Layer Adsorption AndReaction (SILAR). Next, a top electrode 404 is formed on the n-typesemiconducting layer 402. As highlighted above, the substrate 202 (ifinherently conducting) or the layer 204 of conductive material serves asa bottom electrode of the device. Top electrode 404 is formed from atransparent conductive material, such as doped zinc oxide (ZnO),indium-tin-oxide (ITO), doped tin oxide or carbon nanotubes. The processfor forming an electrode from these materials would be apparent to oneof skill in the art and thus is not described further herein.

The present invention further provides a photovoltaic module whichincludes a plurality of electrically interconnected photovoltaic devicesdescribed in the present invention. In particular, the module may bemonolithically integrated using processes well known to those skilled inthe art (as an example, see M. Powalla et al., “Large-Area CIGS Modules:Pilot Line Production and New Developments,” International PVSEC-14,Bangkok, Thailand, Jan. 26-30, 2004; IN28V/18-1, the entire contents ofwhich are incorporated by reference herein).

The present techniques are further described by way of reference to thefollowing non-limiting examples.

Example 1

A bulk layer was prepared according to the method described in U.S.Patent Application Publication No. 2011/0094557:

All operations were performed in a nitrogen-filled glove box. A solutionfor depositing the bulk precursor layer was prepared in two parts inglass vials under magnetic stirring: A1, by dissolving Cu₂S, 0.573 grams(g) and sulfur, 0.232 g in 3 milliliters (ml) of hydrazine and B1, bymixing SnSe, 0.790 g, Se, 1.736 g and Zn, 0.32 g with 7 ml of hydrazine.After 3 days under magnetic stirring, solution A1 had an orangetransparent aspect, while solution B1 was dark green and opaque.Solutions A1 and B1 were mixed (to form solution C1) before deposition.

A solution (D1) for depositing a Sn-containing capping layer wasprepared by dissolving 0.12 g SnS and 0.88 g Se in 4 ml of hydrazine.

A bulk precursor film was deposited on a soda lime glass substratecoated with 700 nm molybdenum by spin coating at 800 revolutions perminute (rpm) and heating the precursor film at 425° C. for 1 minute. Thecoating and heating cycle was repeated 5 times.

An over-layer of solution D1 was then spun on the top of the thusobtained bulk layer, thereby forming a capping layer (to a thickness offrom about 300 nm to about 500 nm), and the whole stack (i.e., bulklayer and capping layer) was annealed at 540° C. on a ceramic hot plate,leading to a large-grained film (Sample M, see surface scanning electronmicrograph (SEM) image of Sample M shown in FIG. 6). An image of thisfilm produced using the capping layer is shown in FIG. 6. A controlsample was prepared without Sn-containing over-layer (capping layer)(Sample N, see surface SEM image of Sample N shown in FIG. 5). As willbe described in detail below, FIGS. 7 and 8 are cross-sectional imagesof completed baseline device from Sample N (FIG. 7) and a device withSample M absorber processed with a capping layer (FIG. 8).

Example 2

A bulk layer was prepared the same as in example 1 except for employingless selenium (1.405 g) in the solution B2, mixed with solution A1 toform solution C2. Capping solution E2 contained only selenium (0.880 g)in 4 ml of hydrazine.

Films were then prepared as described in conjunction with thedescription of Example 1, above: Sample P was prepared by spin coating 5layers of solution B2. Sample Q was prepared by spin coating 5 layers ofsolution B2 and a layer of solution E2, containing only selenium. SampleR was prepared by spin coating 5 layers of solution B2 and a layer ofsolution D1, containing selenium and tin.

Example 3

Solar cells with approximate area of 0.45 square centimeters (cm²) werefabricated from the obtained kesterite films sample P type and sample Rtype by deposition of 60 nm CdS buffer layer by chemical bathdeposition, 100 nm insulating ZnO and 130 nm ITO (indium-doped zincoxide) by sputtering. Statistically significant efficiency enhancementof 19% average was found when samples were treated with a capping layer.See FIG. 9. FIG. 9 is an efficiency distribution of all samples of typeP prepared with no capping layer and all samples of type R prepared withSnS—Se capping layer.

A discussion of the example results now follows. FIGS. 5 and 7 presentsurface and cross-section SEM images, respectively, of films without acapping layer, which show grain size in the range of from about 1micrometer to about 2 micrometers. FIGS. 6 and 8 present surface andcross-section SEM images, respectively, of identically processed filmswith SnS—Se capping layer D1 which show larger grain size in the rangeof from about 2 micrometers to about 5 micrometers.

FIGS. 10-12 are SEM images of a film prepared from a solution withreduced selenium content without a capping layer (Sample P, see surfaceSEM image of Sample P shown in FIG. 10), with a Se capping layer (SampleQ, see surface SEM image of Sample Q shown in FIG. 11) and with SnS—Secapping layer (Sample R, see surface SEM image of Sample R shown in FIG.12). The grain size increases in the sequence Sample P (from about 1micrometer to about 2 micrometers)<Sample Q (from about 2 micrometers toabout 3 micrometers)<Sample R (from about 2 micrometers to about 5micrometers).

Sometimes it is desirable to reduce the amount of Se in the solution toreduce either the thickness of the resulting MoSe₂ layer that forms atthe interface between the CZTSSe and Mo layers (more Se in solutionleads to a thicker interfacial layer). Also, reducing the amount of Sein the solution can influence the amount of voiding that is observed atthe interface between the Mo and CZTSSe. Thus, in this example, less Sewas used in the bulk film.

In addition to the enhanced bulk crystallization effect, smaller grainformations are observed on the surface of sample P, to a lesser extenton sample Q and are completely eliminated in Sample R, with the combinedaction of the SnS and Se in the capping layer. Energy dispersivespectrometer (EDS) analysis of a surface of Sample M without a cappinglayer shows non-uniform elemental distribution, including Zn-rich zones(see surface EDS scan of sample M in FIG. 13) while the Sn—Se cappedfilms show more uniform compositional profile (see surface EDS scan ofsample N in FIG. 14). Cross-sectional scans of these samples (seecross-sectional EDS scan of sample M in FIG. 15 and cross-sectional scanof sample N in FIG. 16) do not show significant difference in the bulkcomposition as a result of the Sn-containing capping layer in this case.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

1. A method of fabricating a kesterite film having a formulaCu_(2−x)Zn_(1+y)Sn(S_(1−z) Se_(z))_(4+q), wherein 0≦x≦1; 0≦y≦1; 0≦z≦1;and −1≦q≦1, the method comprising the steps of: providing a substrate;forming a bulk precursor layer on the substrate, the bulk precursorlayer comprising Cu, Zn, Sn and at least one of S and Se; forming acapping layer on the bulk precursor layer, the capping layer comprisingat least one of Sn, S and Se; and annealing the bulk precursor layer andthe capping layer under conditions sufficient to produce the kesteritefilm having values of x, y, z and q for any given part of the film thatdeviate from average values of x, y, z and q throughout the film by lessthan 20 percent.
 2. The method of claim 1, wherein the substratecomprises a metal foil substrate, a glass substrate, a ceramicsubstrate, aluminum foil and a polymer substrate.
 3. The method of claim1, wherein the bulk precursor layer is formed by depositing Cu, Zn, Snand at least one of S and Se all together onto the substrate usingsolution coating, evaporation, electrochemical deposition or sputtering.4. The method of claim 1, wherein the bulk precursor layer is formed bydepositing a stack of layers on the substrate, each of the layerscontaining at least one of the Cu, the Zn, the Sn and the at least oneof S and Se, and wherein each of the layers is deposited using solutioncoating, evaporation, electrochemical deposition or sputtering.
 5. Themethod of claim 3, wherein the bulk precursor layer is formed by thesteps of: forming a solution or dispersion comprising Cu, Zn, Sn andleast one of S and Se in a solvent; and depositing the solution ordispersion on the substrate.
 6. The method of claim 5, wherein thesolution is deposited on the substrate using a solution coating processselected from the group consisting of: spin coating, dip coating, doctorblading, curtain coating, slide coating, spraying, slit casting,meniscus coating, screen printing, ink jet printing, pad printing,flexography, and gravure printing.
 7. The method of claim 1, wherein thebulk precursor layer is formed on the substrate to a thickness of fromabout 100 nanometers to about 5 micrometers.
 8. The method of claim 1,wherein the capping layer is formed on the bulk precursor layer by thesteps of: dissolving at least one of Sn, S and Se in a solvent to forman ink; and depositing the ink on the bulk precursor layer to form thecapping layer.
 9. The method of claim 8, wherein the solvent is selectedfrom the group consisting of hydrazine and hydrazine-water mixtures,with hydrazine content from about 0.1% to about 99.9%.
 10. The method ofclaim 8, wherein the ink is deposited on the bulk precursor layer usinga solution coating process selected from the group consisting of: spincoating, dip coating, doctor blading, curtain coating, slide coating,spraying, slit casting, meniscus coating, screen printing, ink jetprinting, pad printing, flexography, and gravure printing.
 11. Themethod of claim 1, wherein the capping layer is formed on the bulkprecursor layer to a thickness of from about 10 nanometers to about 3micrometers.
 12. The method of claim 1, wherein the capping layercomprises only one of S and Se.
 13. The method of claim 1, wherein theconditions include temperature and duration.
 14. The method of claim 13,wherein the temperature is from about 300° C. to about 700° C.
 15. Themethod of claim 13, wherein the duration is from about 1 second to about24 hours.
 16. The method of claim 1, wherein the bulk precursor layerand the capping layer are annealed in an environment containing at leastone of nitrogen, argon, helium, hydrogen, forming gas, S vapor, Sevapor, Sn vapor, water vapor and oxygen vapor.
 17. A kesterite filmhaving a formula Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q), wherein 0≦x≦1;0≦y≦1; 0≦z≦1; and −1≦q≦1 formed by the method of claim 1 such thatvalues of x, y, z and q for any given part of the film deviate fromaverage values of x, y, z and q throughout the film by less than 20percent.
 18. The kesterite film of claim 17, wherein x, y, z and qrespectively are: 0≦x≦0.5; 0≦y≦0.5; 0≦z≦1; and −0.5≦q≦0.5.
 19. Aphotovoltaic device, comprising: a substrate; a kesterite film absorberlayer having a formula Cu_(2−x)Zn_(1+y)Sn(S_(1−z)Se_(z))_(4+q), wherein0≦x≦1; 0≦y≦1; 0≦z≦1; and −1≦q≦1 formed on the substrate by the method ofclaim 1 such that values of x, y, z and q for any given part of the filmdeviate from average values of x, y, z and q throughout the film by lessthan 20 percent; an n-type semiconducting layer on the kesterite film;and a top electrode on the n-type semiconducting layer.
 20. Thephotovoltaic device of claim 19, further comprising: an electricallyconductive layer on the substrate.
 21. The photovoltaic device of claim19, wherein the substrate comprises one or more of a metal foilsubstrate, a glass substrate, a ceramic substrate, aluminum foil and apolymer substrate.